As a dense wavelength division multiplexing (DWDM) technology is applied in an optical fiber communications system and a data center system, all-optical switching has become a trend for meeting an increasing bandwidth. In a dense wavelength division multiplexing system, different optical wavelengths carry different optical signals, and optical signals of different wavelengths are transmitted in a same optical fiber, so as to implement large-capacity and low-loss data communication. An optical switch is a key component for implementing an all-optical switching system, and can implement functions such as route selection, wavelength selection, optical cross-connect, and self-healing protection at an all-optical layer. Currently, implemented optical switches include a conventional optical switch of a mechanical structure, a micro-electro-mechanical system switch, a liquid crystal optical switch, a waveguide optical switch, and a semiconductor optical amplifier optical switch. The waveguide optical switch is usually prepared on a silicon on insulator (SOI) platform or an indium phosphide (InP) platform by using a mature complementary metal-oxide-semiconductor (CMOS) technology, and a switching speed can reach an order of nanoseconds or microseconds by using a thermo-optical effect or a plasma dispersion effect of a silicon material. In addition, the waveguide optical switch has a small size and high integration, and is compatible with the CMOS technology, so that low-cost mass production can be implemented. A waveguide microring resonator is a wavelength-sensitive selective conduction device, has advantages of a compact structure, high integration, low power consumption, a simple design, and the like, and can be used to implement functions such as filtering, multiplexing, demultiplexing, routing, wavelength conversion, optical modulation, and optical switching. When a wavelength division multiplexing optical signal passes through a microring resonator, if a wavelength of the optical signal conforms to a resonant wavelength of the microring resonator, the optical signal is coupled to the microring resonator to generate resonance, so as to implement a routing function of an optical signal of a specified wavelength. Compared with a silicon-based optical switch matrix of a cascaded Mach Zehnder interferometer (MZI) type, an optical switch array consisting of a microring resonator has a simple topology structure, a few of stages, and has wavelength selectivity. Therefore, an optical signal passing through a wavelength is not affected by coupling of the microring resonator, and a pass-through insertion loss is very low. Particularly, in a metropolitan aggregation ring of a metropolitan optical network, an optical switch of a microring resonator type has both a filter function and signal uploading and downloading function, so that a switching node device is simple and efficient. For ease of description, the microring resonator is referred to as a microring for short on some occasions.
A dynamic wavelength division multiplexing technology is key in a future optical network. In a wavelength division multiplexing system, wavelengths of channels are different, and need to be controlled by a filter. According to a standard of the International Telecommunication Union, a channel spacing is 0.8 nm or 0.4 nm, or even narrower. Such a narrow channel spacing has a higher requirement on tuning accuracy and a filter feature of a tunable microring resonator. Otherwise, optical signals of a plurality of channels may be simultaneously downloaded or uploaded within an operating spectral bandwidth, and consequently severe channel crosstalk is caused. In addition, to enable a channel to be flexibly deployed in an entire operating band (for example, a C band or an L band), a tuning range of the microring resonator needs to be large enough to cover the entire band. Otherwise, free uploading and downloading of all channels in the WDM system cannot be implemented. For a resonant component having a periodic filter feature, and in particular, when the filter feature of the microring resonator presents a periodic comb spectrum, the tuning range is usually limited by a free spectral range (FSR) because the resonant wavelength of the microring resonator meets a feature equation: FSR=λm+1−λm=λm2/ngL, where m is a longitudinal mode order of a mode, λm is a resonant wavelength of an mth order longitudinal mode of the microring resonator, λm+1 is a resonant wavelength of an (m+1)th order longitudinal mode of the microring resonator, ng is a group refractive index of the mode, and L is a perimeter of the microring resonator. To increase the tuning range, the free spectral range FSR of the microring resonator needs to be as large as possible. Currently, there are two main methods to extend the FSR. A first method is to reduce a radius or a perimeter of the microring resonator. It can be learned, according to the feature equation of the microring resonator, that the free spectral range FSR extends as the perimeter L decreases. However, in this method, not only processing difficulty is increased, but also tuning difficulty is increased, and a very high requirement on an external thermal field or an external electric field is imposed for a thermo-optical tuning temperature or electro-optic tuning power, and consequently thermal stability of a component is poor. The other method is to use a cursor effect between microrings with different radiuses to extend the FSR and reduce a passband spectrum side lobe. However, although this method reduces a thermo-optical tuning temperature or electro-optic tuning power, driving control of a plurality of microring resonators is relatively complex.
FIG. 1 is a microring resonator with a wide FSR. The microring resonator is a microring that has a radius of 2.75 μm and that is based on an SOI platform. The microring includes three ports: an input port, a throughput port, and a drop port. An optical signal that conforms to a resonant wavelength of the microring in a channel spectrum and that is input at the input port is coupled to the microring and is output from the drop port, and an optical signal that does not conform to the resonant wavelength of the microring is not coupled to the microring and is directly output from the throughput port.
FIG. 2 is a spectrogram of the microring resonator. It can be learned that the FSR is approximately 33.4 nm, and a 3 dB spectral bandwidth is 25 GHz. Therefore, it can be learned that a compact microring resonator can implement an operating bandwidth of a C band with an FSR close to 35 nm, so that free uploading or downloading of all channels of the C band in the WDM system can be implemented.
FIG. 3 is a thermo-optical tuning optical filter of a cascaded microring. A waveguide is designed as a ridge waveguide, a width is 0.4 μm, an inner ridge height is 0.34 μm, and a flat region height is 0.1 μm. At the wavelength of 1550 nm, an effective refractive index is 2.83. To improve tuning precision, if a radius R1 of a microring 1 is selected to be 48 μm, FSR1=2.8 nm. If a radius R2 of a microring 2 is 50 μm, FSR2=2.7 nm. FSR=FSR1−FSR2=2.8−2.7=0.1 nm, and a cursor effect of the cascaded microring resonator is shown in FIG. 4. Initial alignment is started from a wavelength λ00. Assuming that alignment is performed again at a wavelength (λ00+Δλmax) after the microring 1 passes through N resonant peaks and the microring 2 passes through (N+1) resonant peaks, that is, FSR1×N=FSR2×(N+1), it can be learned that N=27. Therefore, a maximum wavelength tuning range is: Δλmax=FSR1×27=75.6 nm. If maximum wavelength tuning is implemented by controlling to heat the microring 2, a tuning wavelength of the microring 2 needs to be moved: FSR×N=2.7 nm, (N+1=28) discrete alignment wavelengths may be obtained in total, and a spacing between these discrete alignment wavelengths is FSR1=2.8 nm. If the microring 1 is controlled to be heated, a total moving amount of the resonant wavelength is 2.7 nm, each moving step is 0.1 nm, and 27 comb spectrums may be obtained in total. For each fixed comb spectrum, 28 discrete alignment wavelengths may be obtained by heating the microring 2. Therefore, if the microring 1 and the microring 2 are simultaneously heated, (M=27×28=756) discrete wavelengths can be obtained in total, and a spacing between adjacent wavelength is: Δλmax/756=0.1 nm.
It can be learned from the above that, in the first microring resonator control method, a wider resonant wavelength tuning range of a single microring indicates greater tuning difficulty, and a very high requirement is imposed on the thermo-optical tuning temperature or electro-optic tuning power. Consequently, thermal stability of the component is poor. In addition, a smaller size of a unit microring indicates a higher requirement on a processing technology. In the second microring resonator control method, it is complex to control double microring resonators. In other words, there is no simple microring resonator control method with low drive power in the prior art.